Image processing apparatus and image processing method

ABSTRACT

According to one embodiment, an image processing apparatus includes a scanner to generate image data by scanning a document and a control unit. The control is configured to set, for each pixel in the image data, a second pixel value based on a first pixel value and a difference between a first base value corresponding to a base color of the document and a second base value that is based on neighboring pixel values around the pixel. A correction table is used to obtain a corrected second pixel value for each pixel. A third pixel value for each pixel is set based on the corrected second pixel value and the difference between the first and second base values.

FIELD

Embodiments described herein relate generally to an image processingapparatus and an image processing method.

BACKGROUND

Often, when a document has images printed on both sides of a sheet,images from one side may “show through” on the other side when thedocument is read or electronically scanned. In this context,“show-through” is a phenomenon in which an image or text on the side ofthe document being read is marred or obscured by an image or text on theother side the document. One known technique for removing or correctingfor “show-through,” is a technique of performing a background removalprocessing on the image data generated via a reading process. Thebackground removal process is a process of converting gradationinformation so that pixel values in a light density range of the imagedata approaches a white pixel data value. Here, a “light density range”refers to a relatively low image density area, such as would be presentin a text background portion of an image or a whitespace, background, orthe like, of an image.

However, when a base color (background color) of the document or atleast a portion thereof is not intended to be white, then there will bea region in which the base color will not correspond to the particularlight density range. In such a case, the show-through phenomenon cannotbe corrected in the region of the document having the non-white basecolor by the background removal process.

DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an overall configuration of an image processingapparatus.

FIG. 2 depicts a hardware configuration of an image reading unit.

FIG. 3 depicts aspects of a pixel value correction.

FIG. 4 is a flowchart depicting aspects of processes of a local groundlevel detecting unit.

FIG. 5 is a diagram illustrating a reference region of image data.

FIG. 6 is a diagram illustrating a reference region including basecolors having different hues.

FIG. 7 is a flowchart depicting aspects of processes of a gradationcorrecting unit.

FIG. 8 schematically illustrating aspects of a correction processing.

FIG. 9 depicts an example of image data after correction processing.

DETAILED DESCRIPTION

In general, according to one embodiment, an image processing apparatuscomprises a scanner to generate image data by scanning a document. Theimage data from the scanner comprises a plurality of pixels, each pixelin the image data respectively having a first pixel value correspondingto a brightness value of the pixel. A control unit is configured to set,for each pixel in the image data, a second pixel value based on thefirst pixel value of the pixel and a first difference between a firstbase value corresponding to a base color of the document and a secondbase value that is based on neighboring first pixel values of aplurality of neighboring pixels around the pixel. The control unit usesa correction table to obtain, for each pixel in the image data, acorrected second pixel value. The control unit then sets a third pixelvalue based on the corrected second pixel value and the firstdifference.

Hereinafter, an image processing apparatus according to an exampleembodiment is described with reference to drawings.

FIG. 1 is an external view illustrating an overall configuration of animage processing apparatus 100 according to an embodiment. The imageprocessing apparatus 100 is, for example, an image forming apparatussuch as a multi-function printer, a multi-functional peripheral (MFP) orthe like. The image processing apparatus 100 includes a display 110, acontrol panel 120, a printer 130, a sheet accommodating unit 140, and animage reading unit 200. Here, the printer 130 may be anelectrophotographic device that fixes a toner image to a sheet or may bean inkjet device which ejects ink on a sheet.

The image processing apparatus 100 generates digital data by scanning animage on a sheet and generates an image file therefrom. The sheet is,for example, a document, a paper on which text, graphics, or the likehas been printed or otherwise formed. In general, the sheet may be ofany type and/or material as long as the image processing apparatus 100is able to read the sheet.

The display 110 is an image display device such as a liquid crystaldisplay or an organic electro luminescence display. The display 110displays information related to operations and/or functions of the imageprocessing apparatus 100.

The control panel 120 includes a plurality of buttons. The control panel120 receives inputs from a user, such as those accompanying pressing orselection of one or more buttons of the control panel 120. The controlpanel 120 outputs signals to a control unit of the image processingapparatus 100 according to inputs performed by the user. The display 110and the control panel 120 may be configured as an integrated touch panelin some examples.

The printer 130 forms an image on a sheet based on image informationgenerated by the image reading unit 200 or image information receivedvia a network communication path or the like. In some examples, an imageforming unit of the printer 130 forms an electrostatic latent image on aphotoconductive drum based on the image information generated orreceived. The image forming unit then forms an initial image byattaching a developer onto the electrostatic latent image. A specificexample of the developer includes toner. An image transfer unit of theprinter 130 then transfers the initial image to the sheet. A fixing unitof the printer 130 fixes the image on to the sheet by heating andapplying pressure. The sheet on which the image is formed may beaccommodated in the sheet accommodating unit 140 or may be a sheetchosen by the user.

The sheet accommodating unit 140, in general, accommodates sheets to beused by the printer 130 in printing processes.

The image reading unit 200 reads image information from an originaldocument by contrast of light. The image reading unit 200 records theimage information that has been read from an original document. In someinstances, the recorded image information may be transmitted to anotherinformation processing apparatus via a network connection. Afterreading, the recorded image information can also then be printed on asheet by the printer 130.

FIG. 2 is a diagram illustrating an example of a hardware configurationof the image reading unit 200 according to an embodiment.

The image reading unit 200 includes a scanner 201, a central processingunit (CPU) 202, and a dynamic random access memory (DRAM) 203. The imagereading unit 200 further includes an image compressing unit 206, anexternal interface 207, and a graphics processing unit (GPU) 208. Eachof these components is connected through a bus 209. The CPU 202 and theGPU 208 may cooperate to execute a program stored in a memory to realizea local ground level detecting unit 204 and a gradation correcting unit205. In some examples, the CPU 202 may operate alone (i.e., without GPU208) to execute a program to realize the local ground level detectingunit 204 and the gradation correcting unit 205.

The scanner 201 generates image data by scanning a document or the like.In the present embodiment, the scanner 201 generates image dataexpressed in an RGB (Red, Green, Blue) color space.

Although not specifically illustrated, the scanner 201 may include acharge-coupled device (CCD) sensor, a scanner lamp, a scanning opticalsystem, a condensing lens, and the like. A CCD sensor can be coveredwith the three types of RGB filters, convert received light into anelectric signal, and generate image data comprising a color value foreach RGB color for a plurality of pixels. The scanner lamp illuminatesan image on the document to be read. The scanning optical system ismounted with a mirror for changing an optical path of reflected lightfrom the document being read. The condensing lens focuses the reflectedlight from the document.

The CPU 202 stores image data output from the scanner 201 in the DRAM203 and outputs the image data to the local ground level detecting unit204. The local ground level detecting unit 204 obtains a local groundlevel value of each pixel of the image data. The local ground levelvalue indicates a base color of a region proximate to the pixel. In thiscontext, a base color is the color of a region that does not include aportion of an illustration, a text character, or the like. That is, ingeneral, the base color refers to the color of a background region ofthe read document data. Details about processes of the local groundlevel detecting unit 204 will be described later with reference to FIG.4.

The CPU 202 outputs the image data processed by the local ground leveldetecting unit 204 to the gradation correcting unit 205. The gradationcorrecting unit 205 adjusts each pixel value of the image data based onthe local ground level value obtained by the local ground leveldetecting unit 204. The gradation correcting unit 205 performs gradationconverting based on a pixel value correction table or the like. Detailsabout processes of the gradation correcting unit 205 will be describedlater with reference to FIG. 7.

FIG. 3 is a diagram illustrating aspects of a pixel value correctionmethod. FIG. 3 illustrates a pixel value correction method when thecolor space of image data is RGB. While FIG. 3 depicts a pixel valuecorrection method using an XY graph, the correction method may beimplemented as a corresponding correction table or other lookup method.The horizontal axis of FIG. 3 represents an input pixel value (inputvalue) and the vertical axis represents a corrected pixel value (outputvalue). In general, correction would be performed independently for eachof red, green, and blue color values.

The correction illustrated in FIG. 3 has a correspondence relationshipin which an output pixel value (corrected pixel value) becomes largerthan an input pixel value for those pixel values included in the lightdensity range. When the color space of image data is standard RGB, thelight density range is the range in the which a gradation value of thepixel is close to 255. The correction does not substantially changeoutput value of an input pixel that is outside the light density range(that is, not near a gradation value of 255).

Based on the correction illustrated in FIG. 3, a pixel value of a pixelin the light density range is corrected such that the output pixel valueis increased relative to the input value. In other words, a bright imageapproaches a white background. As a result, a bright image generated ina white background region of the image data is removed, and ashow-through image is removed or reduced.

Here, as illustrated in FIG. 3, the labeled pixel values 501 and 502 arenot included in the light density range. The labeled pixel values 501and 502 correspond to colors having different hues. Thus, when a basecolor of the image data is not a white background but is rather a colorhue corresponding to the labeled pixel value 501 or 502, or the like, aoutput pixel value is not changed from the input pixel value.Accordingly, when a show-through occurs in a region of a non-white basecolor, the show-through image would not be removed or reduced. Thus, inthe present disclosure, a pixel correction is performed according to thelocal ground level detecting unit 204 and the gradation correcting unit205.

The CPU 202 outputs image data that has been processed by the gradationcorrecting unit 205 to the image compressing unit 206. The imagecompressing unit 206 performs a compression on the image data ascorrected by the gradation correcting unit 205. The image compressingunit 206 converts the image data to, for example, a Joint PhotographicExperts Group (JPEG) format or the like. The compressed image can betransmitted to an external device, a hard disk, or the like via theexternal interface 207.

In some examples, the local ground level detecting unit 204, thegradation correcting unit 205, and the image compressing unit 206 areconfigured as a hardware component or the like. For example, a hardwarecomponent is a graphics processing unit (GPU), field programmable gatearray (FPGA), or the like.

Processes of the local ground level detecting unit 204 are describedbelow with reference to FIGS. 4 through 6. Processes of the gradationcorrecting unit 205 are described with reference to FIGS. 7 and 8. Inthe present example, a color space of image data being processed is RGB.

FIG. 4 is a flowchart illustrating the processes of the local groundlevel detecting unit 204. The local ground level detecting unit 204selects a target pixel from the pixels in image data. For example, thelocal ground level detecting unit 204 sequentially selects as a targetpixel each pixel along a raster scan direction of the image data.

The local ground level detecting unit 204 sets a reference region forthe target pixel (ACT 201). The reference region is a region surroundingthe target pixel within a certain range from the target pixel. Thereference region includes a plurality of pixels positioned around thetarget pixel.

FIG. 5 is a diagram illustrating a reference region 303 in the imagedata 301. As described above, the local ground level detecting unit 204sequentially selects a target pixel 302 from the pixels in the imagedata along a raster scan direction 304.

Referring to FIG. 5, the reference region 303 is a region including a7×7 pixel group centered on the particular target pixel 302. The localground level detecting unit 204 changes the 7×7 pixel group according tothe particularly selected target pixel 302.

In general, the size of the reference region 303 is not limited to 7×7.The size of the reference region 303 may be set according to the imagetype of the image data 301. For example, the local ground leveldetecting unit 204 may change the size of the reference region 303according to whether the image data corresponds to a photograph, adocument, or other type.

Alternatively, the local ground level detecting unit 204 may set thesize of the reference region 303 according to a detected contrast levelin image data. For example, the reference region 303 for the image data301 having a high contrast can be set to be a small size (e.g., lessthan 7×7). On the other hand, the reference region 303 for the imagedata 301 having a low contrast can be set to be a large size.

A reference region 303 for a target pixel 302 positioned at or near anedge of the image data 301 might have to extend beyond the edge of theimage to have its set size (e.g., 7×7). However, in such a case, thelocal ground level detecting unit 204 may simply omit any portion of thereference region 303 that would otherwise be outside (beyond an edge of)the image data 301.

Referring back to FIG. 4, the local ground level detecting unit 204obtains a local ground level value for each RGB color for the targetpixel (ACT 202 to ACT 204). In other words, the local ground leveldetecting unit 204 obtains local ground level values ‘Rmax’, ‘Gmax’, and‘Bmax’.

A local ground level value of a target pixel is based on each pixelvalue in a reference region. In other words, a local ground level valuecorresponds to a base color of the reference region. In this context,abase color is, for example, a background region color that is differentfrom a color corresponding to a portion of illustration, a textcharacter, or the like. The background regions tend to have highbrightness compared to the regions of illustration, text character, orthe like. Thus, the local ground level detecting unit 204 typicallyobtains a pixel value having a high brightness as the local ground levelvalue.

The reference region may also include pixels having base colors withdifferent hues. In this case, the local ground level value may be avalue close in value to the target pixel. Here, processes for obtainingthe local ground level value when the reference region includes aplurality of base colors having different hues are described withreference to FIG. 6.

FIG. 6 is a diagram illustrating a reference region 303 including aplurality of types of base color having different hues. The referenceregion 303 of FIG. 6 includes two base colors 601 and 602 havingdifferent hues from each other. Here, the base color 602 has higherbrightness than the base color 601. In this example, the determinedlocal ground level value corresponds to a pixel value of the base color602. However, in this instance, the hue of the target pixel 302 iscloser to the base color 601 than to the base color 602. Thus, it wouldbe preferable that the local ground level value be set to the base color601 instead of base color 602.

Accordingly, the local ground level detecting unit 204 is configured toset the local ground level value to a pixel value having the higherbrightness when the difference between the initially calculated localground level and the target pixel is less than or equal to a firstthreshold value. In the example of FIG. 6, the difference between thepixel value of the target pixel and the pixel value corresponding to thebase color 602 is greater than the first threshold value. Thus, thepixel value of the base color 601, which has a difference between thepixel value of the target pixel that is within the first threshold valueand a brightness that is higher than the target pixel is set as thelocal ground level value.

Referring back to FIG. 4, the local ground level detecting unit 204obtains a maximum value for R (red) level (Rmax) among the referenceregion 303 pixels and when the difference between the maximum value(Rmax) and the R level of the target pixel 302, the local ground levelvalue is set to Rmax (ACT 202). The local ground level detecting unit204 then preforms a similar processing on the reference region 303 forthe G (green) level (Gmax) (ACT 203) and a similar processing on thereference region 303 for B (blue) level (Bmax) (ACT 204).

The local ground level detecting unit 204 increments the position of thetarget pixel along the raster scan direction (ACT 205). The local groundlevel detecting unit 204 determines whether the newly selected targetpixel is within the range of image data (ACT 206). When the target pixelis within the range of image data (YES in ACT 206), processes from ACT201 to ACT 205 are repeated for the new target pixel.

For each target pixel, the pixel(s) having the local ground level valuefor each RGB color might not be the same pixel. That is, for the sametarget pixel, pixel values of different pixels among the referenceregion may correspond to the determined local ground level value for thedifferent RGB colors. Also, in this processing the first threshold valueused with each RGB color may be different according to the RGB color.

FIG. 7 is a flowchart illustrating processes of the gradation correctingunit 205. The gradation correcting unit 205 obtains ground level values‘Rgnd’, ‘Ggnd’, and ‘Bgnd’ for an entire document/image (ACT 401). Aground level value of the entire document/image (hereinafter, referredto as an overall ground level value) is, for example, the color of thesheet being printed or a color of margin area of the sheet.

The overall ground level value may be a fixed value or may be obtainedbased on analysis of pixel values in image data. When the overall groundlevel value is obtained based on the pixel values in image data, thehighest detected pixel brightness among the pixel values in image datacan be set to the overall ground level value, for example.

When a color space of the image data is RGB, the overall ground levelvalue is generally close to a RGB value 255, 255, 255. In the exampleembodiment, a case in which the overall ground level value is (Rgnd,Ggnd, Bgnd)=(254, 254, 254) is utilized.

The gradation correcting unit 205 selects a target pixel. The gradationcorrecting unit 205 obtains pixel values ‘R’, ‘G’, and ‘B’ for thetarget pixel and local ground level values ‘Rmax’, ‘Gmax’, and ‘Bmax’(ACT 402). The local ground level values ‘Rmax’, ‘Gmax’, and ‘Bmax’ forthe target pixel are obtained according to the flowchart of FIG. 4.

Next, the gradation correcting unit 205 obtains base offset values‘Rofs’, ‘Gofs’, and ‘Bofs’ for the target pixel according to processesACT 403 to ACT 409. The base offset values are respective differencesbetween the overall ground level values ‘Rgnd’, ‘Ggnd’, and ‘Bgnd’ andthe local ground level values ‘Rmax’, ‘Gmax’, and ‘Bmax’.

The gradation correcting unit 205 calculates a difference (Rgnd−R) bysubtracting a pixel R value from the overall ground level value Rgnd.The gradation correcting unit 205 determines whether the calculateddifference is less than or equal to a second threshold value (ACT 403).

When the difference is greater than the second threshold value (NO inACT 403), the color density of the target pixel is considered high. Inthis case, it may be determined that the target pixel is highly likelyto correspond to an illumination or character region. The show-throughphenomenon is more likely to occur in a region having lower colordensity or higher brightness. On the other hand, show-through hardlyoccurs in regions having high density or low brightness. Accordingly,when the target pixel has high density, the target pixel may bedetermined to be a pixel that is hard to show through.

Thus, when the difference is greater than the second threshold value,the target pixel is excluded from gradation converting. In particular,the gradation correcting unit 205 updates the local ground level valueRmax. In other words, the local ground level value Rmax is replaced bythe overall ground level value Rgnd (ACT 404).

Also, as a result, the base offset value Rofs calculated in process ACT409 is set to a value ‘0’. When the base offset value has the value ‘0’,adjusting of a pixel value in process ACT 410 is not performed and thebase offset value is excluded from the gradation converting. Details ofthe gradation converting will be described in detail below.

On the other hand, when the difference is less than or equal to thesecond threshold value (YES in ACT 403), the gradation correcting unit205 does not update the local ground level value Rmax. In other words,the local ground level value Rmax obtained in ACT 202 (FIG. 4) is used.Similarly to the R values, the gradation correcting unit 205 updates alocal ground level value according to a difference also for G values anda B values (ACT 405 to ACT 408). The second threshold value utilized inthe processing may be different for each RGB color.

The gradation correcting unit 205 calculates the base offset values‘Rofs’, ‘Gofs’, and ‘Bofs’ based on the local ground level value and theoverall ground level value (ACT 409). The gradation correcting unit 205calculates the base offset value by subtracting the overall ground levelvalue from the local ground level value.

In particular, the gradation correcting unit 205 obtains the base offsetvalue Rofs=Rmax−Rgnd. The gradation correcting unit 205 obtains the baseoffset value Gofs=Gmax−Ggnd. The gradation correcting unit 205 obtainsthe base offset value Bofs=Bmax−Bgnd.

Here, the local ground level values are (Rmax, Gmax, Bmax)=(230, 230,230). As described above, the overall ground level values in thisexample are (Rgnd, Ggnd, Bgnd)=(254, 254, 254) and the values (Rofs,Gofs, Bofs)=(−24, −24, −24) are thusly calculated as the base offsetvalues.

As such, when a color space of image data is an additive mixture typesuch as RGB or the like, an overall ground level value shows a valuehigher than a local ground level value. Thus, the calculated base offsetvalues are negative values. On the other hand, when a color space ofimage data is of a subtractive mixture type such as CMYK (cyan, magenta,yellow, black) or the like, the calculated base offset values becomepositive values.

The base offset value of each RGB color for the target pixel is obtainedthrough processes ACT 403 to ACT 409. When the difference between theoverall ground value and the target pixel value is greater than thesecond threshold value (NO in ACT 403, ACT 405, and ACT 407), the localground level value is replaced by the overall ground level value. Thus,since the local ground level value and the overall ground level valueare the same after such a replacement, the base offset value has a value‘0’.

Next, the gradation correcting unit 205 calculates normalized pixelvalues ‘Rnrm’, ‘Gnrm’, and ‘Bnrm’ (ACT 410). A normalized pixel valueindicates a value obtained by removing a fixed value of a base colorfrom a pixel value. In other words, a normalized pixel value indicates adifferential color value from which a fixed value of a base color hasbeen removed for a pixel. When show-through occurs in a target pixel,the pixel value of the target pixel includes a color of a show-throughimage.

The gradation correcting unit 205 calculates the normalized pixel valuebased on the pixel value of the target pixel and the base offset value.In particular, the gradation correcting unit 205 obtains the normalizedpixel value by subtracting the base offset value from the pixel value.

The gradation correcting unit 205 obtains the normalized pixel valueRnrm=R−Rofs. The gradation correcting unit 205 obtains the normalizedpixel value Gnrm=G−Gofs. The gradation correcting unit 205 obtains thenormalized pixel value Bnrm=B−Bofs.

A case in which a pixel value of the target pixel is ‘220, 215, or 230’is explained. As described above, the base offset value is (Rofs, Gofs,Bofs)=(−24, −24, −24). The gradation correcting unit 205 calculates avalue 244 {=220−(−24)} as a normalized pixel R. The gradation correctingunit 205 calculates a value 239 {=215−(−24)} as a normalized pixel Gvalue. The gradation correcting unit 205 calculates a value 254{=230−(−24)} as a normalized pixel B value. As such, the normalizedpixel values (Rnrm, Gnrm, Bnrm)=(244, 239, 254) are calculated byremoval of the base color.

On the other hand, when the color space of image data is a subtractivemixture type, the base offset value is a positive value. Accordingly,the normalized pixel values will have a value smaller than the pixelvalues due to the subtracting of the (positive) base offset value fromthe pixel value.

The gradation correcting unit 205 utilizes the normalized pixel valuesas input values to a correction table, a correction function, or thelike, and thereby obtains corrected pixel values ‘Rx’, ‘Gx’, and ‘Bx’ asoutput values (ACT 411). This correction process has been describedabove with reference to FIG. 3. As described above, the correctionrelies on a correspondence relationship in which a gradation value of apixel is corrected to a higher value in a light density range.

In the present embodiment, the gradation correcting unit 205 uses thenormalized pixel value as input for the correction process instead ofthe pixel value of the target pixel. In other words, the gradationcorrecting unit 205 uses the normalized pixel value (from which the basecolor has already been removed) as an input value for the correctionprocess.

Since the base color is already removed from the pixel value beingcorrected, a pixel value of a background region can be considered tocorrespond to the light density range. In addition, when a show-throughoccurs in the target pixel, the pixel value of the target pixelindicates a color of a show-through image due to removal of the basecolor.

Accordingly, the normalized pixel value is corrected to a highergradation value according to the gradation converting. As a result, acolor of a bright image in the background region, that is, a likelycolor of the show-through image, is removed. In this manner, even whenthe base color of the target pixel is not white, the show-through imagemay still be removed or reduced.

For a pixel having high density, that is, a pixel whose base offsetvalue is ‘0’, a normalized pixel value having the same value as thepixel value is used in the correction process. In this case, since thenormalized pixel value does not correspond to the light density range,the output value has the same value as the input value. In other words,the normalized pixel value is excluded from the gradation converting. Assuch, a pixel having high density where show-through hardly occurs isexcluded from the gradation converting.

As described above, the normalized pixel values obtained in process ACT410 are values (Rnrm, Gnrm, Bnrm)=(244, 239, 254). The corrected pixelvalues when these normalized pixel values are input to the correctionprocess (see FIG. 3) are values (Rx, Gx, Bx)=(254, 249, 254). As such,the corrected pixel values have high gradation values.

In process ACT 411, the gradation correcting unit 205 calculates outputpixel values ‘Ry’, ‘Gy, and ‘By’ based on the corrected pixel values andthe base offset values. In particular, the gradation correcting unit 205calculates the output pixel values by adding the corrected pixel values‘Rx’, ‘Gx,’ and ‘Bx’ to the base offset values ‘Rofs’, ‘Gofs’, and‘Bofs’. In this manner, an output pixel value in which the base colorhas been restored to its original value with respect to the correctedpixel value is calculated.

The gradation correcting unit 205 obtains the output pixel valueRy=Rx+Rofs. The gradation correcting unit 205 obtains the output pixelvalue Gy=Gx+Gofs. The gradation correcting unit 205 obtains the outputpixel value By=Bx+Bofs.

As described above, the base offset values are (Rofs, Gofs, Bofs)=(−24,−24, −24). Accordingly, the gradation correcting unit 205 calculates avalue 230 {=254+(−24)} as the output pixel R value. The gradationcorrecting unit 205 calculates a value 225 {=249+(−24)} as the outputpixel G value. The gradation correcting unit 205 calculates a value 230{=254+(−24)} as the output pixel B value.

Accordingly, the output pixel values (Ry, Gy, By)=(230, 225, 230) inwhich a show-through image has been removed or reduced are calculated.Also, in this example, the pixel RGB values of the target pixel arevalues 220, 215, 230, respectively. As such, in this instance, a pixelnot having a white background is corrected by the gradation convertingaccording to the correction processing.

When the color space of image data is a color space of a subtractivemixture, the base offset value is a positive value. Accordingly, theoutput pixel value has a value greater than the corrected pixel valuesince the (positive) base offset value is added to the corrected pixelvalue.

The gradation correcting unit 205 increments the position to the nexttarget pixel in the raster scan direction (ACT 412). The gradationcorrecting unit 205 determines whether a newly selected pixel is withinthe range of the image data (ACT 413). When the target pixel is withinthe range of image data (YES in ACT 413), the gradation correcting unit205 repeats ACT 402 to ACT 412.

FIG. 8 is a diagram for explaining aspects of the correction processingof the present embodiment. As described with reference to the flowchartof FIG. 7, the normalized pixel value from which a base color has beenremoved from the pixel value is input to the correction process. Ashaded region in the graph on the righthand side of FIG. 8 depicts arange in which pixel values will be shifted by removal of the basecolor. As such, the removal of the base color indicates that thecorrection is substantially shifted in a direction indicated by arrow503 in a conceptual manner at least.

According to the graph on the righthand side, the pixel values 501 and502 now correspond to the light density range after the conceptualshifting of the correction table. As a result, the pixel values 501 and502 are targets of the gradation converting. That is, the pixel values501 and 502 become the targets of the gradation converting after theremoval of the base color therefrom.

FIG. 9 is a diagram illustrating an example of image data after acorrection according to the present embodiment. Image data 301 a isimage data before the correction and image data 301 b is image dataafter the correction.

In the example of FIG. 9, the relevant background color of the imagedata 301 a is not a white background. Also, the density of a color 702corresponding to a show-through image is higher than that of the basecolor 701 of the image data 301 a. In this case, it is necessary toaccount for the base color 701 in order to remove the show-throughimage.

In this regard, in the present embodiment, the base color 701 is removedfrom the image data 301 a, and then a gradation converting is performedto remove/reduce the show-through image. After the gradation converting,the base color 701 is added back to each relevant pixel value.Accordingly, as illustrated in the image data 301 b, the base color 701is maintained even though the color 702 of the show-through image may beclose to the base color 701.

As described above, the image processing apparatus according to thepresent embodiment includes a reading unit, such as the scanner 201),and a control unit, such as the CPU 202 and the GPU 208. The readingunit generates image data by scanning a document or the like. Thecontrol unit performs processes on each of a plurality of pixelsincluded in image data.

In general, a control unit obtains a normalized pixel value according toa base offset value and a pixel color value of a target pixel. The baseoffset value is a difference between an overall ground level valueindicating a base color of the document and a local ground level value,which is a value based on each of several neighboring pixel valuesaround the target pixel.

The control unit obtains a corrected pixel value corresponding to thenormalized pixel value based on a correction table or the like. Also,the control unit obtains an output pixel value based on the correctedpixel value and the base offset value.

As such, the image processing apparatus obtains the base offset valueindicating a base color of the pixel. The image processing apparatusobtains the normalized pixel value from which the base color is removedfrom the pixel value using the base offset value. The image processingapparatus performs a correction using the normalized pixel value as aninput to the correction table.

As a result, gradation converting may be performed even on image datahaving a base color that is not white by first removing the base colorfrom the image data. Accordingly, even when a base color is a colorother than white, a color of a show-through image of the image data mayalso approach the base color. Therefore, the show-through image may beprevented or reduced.

Also, the image processing apparatus obtains the local ground levelvalue according to neighboring pixel values of a plurality ofneighboring pixels of a target pixel. In particular, the imageprocessing apparatus obtains a neighboring pixel value whose brightnessis higher than the target pixel value as the local ground level value.Accordingly, a base color surrounding the target pixel may beappropriately obtained.

The local ground level value may ultimately be set to a neighboringpixel value having a brightness greater than the target value, or aneighboring pixel value for which the difference between the pixel valueof the target pixel and the neighboring pixel value is less than orequal to a predetermined threshold value. Accordingly, even when thesurrounding region of the target pixel includes base colors of aplurality of hues, the local ground level value may be appropriatelyobtained even when the base color hue is close to the target pixel.

Modified Example

The image processing apparatus may generate low resolution image datafrom the image data, and obtain a local ground level value according tothe low resolution image data. In this case, the image processingapparatus may obtain pixel values for a plurality of low resolutionpixels in the low resolution image data, which correspond to positionsof surrounding the target pixel in the image data. The image processingapparatus may obtain a local ground level value based on an evaluationof the low resolution pixels corresponding in general to positions ofpixels neighboring the target pixel in the image data.

The number of pixels of the low resolution image data is smaller thanthat of the original image data. Thus, the image processing apparatus isable to obtain a local ground level value using smaller number ofpixels. Accordingly, the local ground level value may be obtained moreefficiently and/or at a high speed.

In the above example embodiments, the processes of the control unit aresubstantially realized by hardware, but the disclosure is not limitedthereto. The various processes of the control unit may be realized bysoftware in whole or in part. A CPU 202 executes a program stored in amemory, such as ROM or the like, to implement the processes of the localground level detecting unit 204 and the gradation correcting unit 205.

While certain embodiments have been described, these embodiments havebeen presented by way of examples only, and are not intended to limitthe scope of the present disclosure. Indeed, the novel embodimentsdescribed herein may be embodied in a variety of other forms; andfurthermore, various omissions, substitutions, and changes in the formof the embodiments described herein may be made without departing fromthe spirit of the present disclosure. The accompanying claims and theirequivalents are intended to cover such forms and modifications as wouldfall within the scope and spirit of the present disclosure.

What is claimed is:
 1. An image processing apparatus, comprising: ascanner to generate image data by scanning a document, the image datacomprising a plurality of pixels, each respectively having a first pixelvalue; and a control unit configured to: set, for each pixel in theimage data, a second pixel value based on the first pixel value of thepixel and a first difference between a first base value corresponding toa base color of the document and a second base value that is based onneighboring first pixel values of a plurality of neighboring pixelsaround the pixel, use a correction table to obtain, for each pixel inthe image data, a corrected second pixel value, and set a third pixelvalue based on the corrected second pixel value and the firstdifference.
 2. The image processing apparatus according to claim 1,wherein the control unit sets a neighboring pixel value having abrightness for each color that is highest among neighboring pixel valuesas the second base value.
 3. The image processing apparatus according toclaim 1, wherein the control unit sets a neighboring pixel value havinga brightness that is highest and for which a difference between theneighboring pixel value and the first pixel value is less than or equalto a first threshold value as the second base value.
 4. The imageprocessing apparatus according to claim 1, wherein the control unit setsthe third pixel value for the pixels for which a difference between thefirst pixel value and the first base value is less than or equal to asecond threshold value.
 5. The image processing apparatus according toclaim 1, wherein the control unit sets the second pixel value bysubtracting the first difference from the first pixel value, the firstdifference being obtained by subtracting the first base value from thesecond base value, and the control unit sets the third pixel value byadding the first difference to the corrected second pixel value.
 6. Theimage processing apparatus according to claim 5, wherein the firstdifference is a negative value when the first pixel value is expressedaccording to a color space of an additive mixture and a positive valuewhen the first pixel value is expressed according to a color space of asubtractive mixture.
 7. The image processing apparatus according toclaim 1, wherein the pixel values before correction having a brightnessequal to or greater than a certain value are associated in thecorrection table to a pixel value after correction having a highestbrightness.
 8. The image processing apparatus according to claim 1,wherein the control unit is configured to: generate low resolution imagedata based on the image data, and obtain the second base value based ona plurality of low resolution pixels from the low resolution image dataat positions corresponding to the plurality of neighboring pixels. 9.The image processing apparatus according to claim 1, wherein the imagedata is RGB data.
 10. The image processing apparatus according to claim1, wherein each pixel in the image data has a pixel value for each of aplurality of different color channels.
 11. An image processingapparatus, comprising: a scanner to generate image data by scanning adocument, the image data comprising a plurality of pixels, eachrespectively having a first pixel value, the document having a firstbase value corresponding to a background color of the document; and acontrol unit configured to: set, with respect to each pixel included inthe image data, a second pixel value by subtracting a first differencefrom the first pixel value of each respective pixel, the firstdifference being obtained by subtracting the first base value from asecond base value corresponding to neighboring pixel values of aplurality of neighboring pixels around the respective pixel, use acorrection table to obtain a corrected second pixel value correspondingto the second pixel value, and set a third pixel value for eachrespective pixel by adding the first difference to the corrected secondpixel value.
 12. The image processing apparatus according to claim 11,wherein the correction table matches pixel values brighter than acertain brightness value to a highest brightness pixel value.
 13. Theimage processing apparatus according to claim 11, wherein the image datais RGB data.
 14. The image processing apparatus according to claim 11,wherein the control unit is configured to convert the image data fromthe scanner to lower resolution image data having fewer total pixels anddetermine the second base value using the lower resolution image data.15. The image processing apparatus according to claim 11, wherein thecontrol unit is configured to calculate a corrected pixel value for eachcolor channel in the image data.
 16. An image processing method,comprising: generating image data by scanning a document, the image datacomprising a plurality of pixels, each pixel respectively having a firstpixel value corresponding a brightness of the pixel, the document havinga base color; setting a second pixel value for each pixel based on thefirst pixel value of the pixel and a first difference between a documentbase value corresponding to the base color of the document and a localbase value corresponding to pixel values of a plurality of neighboringpixels of the pixel; obtaining a corrected second pixel valuecorresponding to the second pixel value using a correction table; andsetting a third pixel value for each pixel based on the corrected secondpixel value of the pixel and the first difference.
 17. The imageprocessing method according to claim 16, further comprising: generatinglower resolution image data from the image data, the lower resolutionimage data having fewer pixels total than the image data; anddetermining the local base value using the lower resolution image data.18. The image processing method according to claim 16, wherein the imagedata is RGB data.
 19. The image processing method according to claim 16,wherein each pixel in the image data has a pixel value for each of aplurality of different color channels.
 20. The image processing methodaccording to claim 16, further comprising: printing an image on a sheetusing the third pixel values for each pixel in the image data.